Many automotive vehicles have systems in place which provide a warning to the operator when an exterior lamp has failed. This can be important in large tractor-trailers, where it may be difficult or impossible for the operator to visually observe that an exterior lamp on the trailer has failed. Traditional systems have been developed which detect the resulting current drop when an incandescent lamp fails (e.g., creates as an open circuit).
More modern vehicles and trailers are increasingly utilizing light emitting diodes (LEDs) as a light source. These LEDs can also fail open in similar fashion as traditional incandescent bulbs. However, a typical LED lamp uses much less current than their incandescent counterparts. Therefore, the current drop due to a failed LED is likely much less than an incandescent bulb and may not be detectable by traditional bulb outage detection systems. For example, traditional incandescent bulb outage detection systems might detect a 2.1 Amp drop in a system when a bulb fails. New LED lamps of the same function can have a drop as little as 0.15 Amps. The issue is complicated when a single tractor may need to tow trailers having either incandescent or LED lamps at different times. The issue is further complicated by the lack of a standard within the U.S. for the reporting or detection of failed lamps.
Hella KG Hueck & Co. has patented a product, shown in German Patent publication DE10215486, which is for LED turn signals. In particular, when the turn signal LEDs are not illuminated, the lamp has an imbedded microchip which actively sends a diagnostic signal through the LED, confirming that the LED is not failed open, and, therefore, presumably operational. A multiplexed signal reporting a detected failure is transmitted to the cab of the truck. This system requires paired electronic modules between the tractor and the trailer, adding complexity to the trailer lighting and harness system.